As the electrophotographic photoreceptor, there has been widely used an inorganic photoreceptor having an inorganic photoreceptive layer comprised of an inorganic photoconductive compound, such as selenium, zinc oxide or cadmium sulfide.
However, these compounds are not necessarily satisfactory in sensitivity, heat stability, moisture resistance and durability. For example, an inorganic photoreceptor having an inorganic photoreceptive layer comprised of selenium is liable to crystallize and deteriorate in photoreceptive properties at a high temperature; therefore, it requires not only a strict temperature control in manufacturing but also a preventive measure against crystallization caused by heat or fingerprints in handling. Further, an inorganic photoreceptor having an inorganic photoreceptive layer comprised of cadmium sulfide or zinc oxide is not satisfactory in moisture resistance and durability.
Under the circumstances, there are actively studied and developed in recent years organic photoreceptors having an organic photoreceptive layer comprised of an organic photoconductive compound.
Japanese Pat. Exam. Pub. No. 10496/1975, for example, discloses an organic photoreceptor having an organic photoreceptive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone, but this organic photoreceptor is not necessarily satisfactory in sensitivity and durability.
To eliminate such defects, there has been developed an organic photoreceptor having a function-separating organic photoreceptive layer in which a carrier (hole or electron) generation function and a carrier transfer function are separately provided by different materials. Such a function-separating organic photoreceptor has an advantage that the materials for the photoreceptive layer can be selected from a wide range of compounds to prepare a photoreceptor having desired properties with ease.
As materials having the carrier generation function, various azo compounds are practically used, and as materials responsible for the carrier transfer function, many compounds are proposed, for example, in Japanese Pat. O.P.I. Pub. Nos. 94829/1976, 72231/1977, 27033/1978, 52063/1980, 65440/1983 and 198425/1983.
However, an organic photoreceptor comprised of such a carrier transfer material is not necessarily satisfactory in carrier transfer property, and when used in a rapid copying process at a low environmental temperature, it causes disadvantages such as deterioration in sensitivity and rise in residual potential. Further, when the simplification of copying process is attempted by decreasing the size of photoreceptor drums, conventional carrier transfer materials have no adequate carrier transferring capability, and thereby decreasing the size of photoreceptor drums inevitably leads to drop in process speed.
In order to solve such problems, there are recently proposed, in Japanese Pat. O.P.I. Pub. Nos. 10747/1986, 269964/1987 and 285552/1988, a technique to use a polysilane having a specific structure as a carrier (hole) transfer material. Unlike conventional carrier transfer materials, such a polysilane has a film-forming property by itself and can readily form a photoreceptive layer in the form of film without being combined with other binders. Moreover, it has a hole mobility of approximately 10.sup.-4 cm.sup.2 /V.multidot.sec or more, which is ten or more times as large as that of conventional carrier transfer materials. Accordingly, the disadvantage inherent in the organic photoreceptor comprised of a conventional carrier transfer material can be removed by use of this material.
However, a photoreceptive layer comprised of this polysilane has other disadvantages such as insufficient flexibility, comparatively low film strength and weak adhesive property. Therefore, even when the polysilane is formed into a film on a flexible belt support, damages such as cracks and peeling are liable to occur, and such damages advance further in course of repetition of the electrophotographic process. In addition, the rubbing force exerted by a toner in the developing process or by a cleaning member in the cleaning process causes surface damages or peeling of the photoreceptive layer, and these damages produce defects such as white lines and black lines at an early stage of the copying process and thereby deteriorate the quality of copied images. This phenomenon is frequently observed when the size of the drum is decreased or the curvature of belt substrates is increased for the simplification of the copying process.